The invention relates to a compacting device operated with vibration oscillations for molding and compacting molding materials in mold cavities of molding boxes to form molded bodies and to a method of using the compacting device, the molded bodies having an upper side and an underside, via which the compacting forces are introduced. In the case of this method, before the compacting operation, the molding material is located in the mold cavities initially as a volume mass of loosely coherent granular constituents, which are molded into solid molded bodies only during the compacting operation by the action of compacting forces on the upper side and underside. When the compacting device is used in machines for producing finished concrete products (for example paving blocks), the volume mass may consist for example of moist concrete mortar. In the case of the compacting devices operating with vibrators for producing finished concrete products, a distinction can be drawn between 3 known generic types, which are suitable for describing the prior art of interest here and which have in common the fact that the molding box and the molding material are arranged on the upper side of a pallet or a base plate during the compacting operation. In this case, during the main compaction a pressing plate which can be moved in the vertical direction by a pressing device and can be driven to exert a predetermined pressing pressure rests on the upper side of the molding material.
The first generic type concerns the popular “conventional type”, known to a person skilled in the art, of impact compaction, in which the vibrating table of a vibrator, which can be regulated with respect to its oscillating stroke amplitude, strikes once against the pallet from below with every oscillating period. This generic type represents the closest prior art, described by EP 0 515 305 B1. It is also the case with the second generic type, the compacting device of which operates very differently than in the case of the first generic type, that the compacting energy originally generated by the vibrator is introduced into the molding material by means of impact processes. In this case, the pallet and the molding box are clamped to the vibrating table during the compacting operation, so that their masses are considered to belong to the mass of the oscillating system and oscillate along with it. The impact point, which can be defined by the colliding of different masses at different velocities, here lies on the upper side and underside of the molding material itself, an air gap being produced during the compaction between the underside of the molded body and the pallet on the one hand and the upper side of the molded body and the pressing plate on the other hand. This second generic type, described by DE 44 34 679 A1, can be described most accurately as a compacting device for carrying out a “shaking compaction”. In the case of the third generic type, documented by EP 0 870 585 A1, the masses of the molding material, the molding box, the pallet and the vibrating table together form a system of masses which represents the oscillating mass of a mass-spring system operating with harmonic (sinusoidal) oscillating movements. The dynamic forces introduced on the upper side and underside of the molded body, which are derived from the oscillating accelerations of the co-oscillating masses, generate a likewise sinusoidally proceeding dynamic compaction pressure (harmonic compaction). Some particulars of interest here on the prior art according to EP 0 515 305 B1 and EP 0 870 585 A1 can also be found in an article in the specialist journal “BFT”, September 2000 edition, pages 44–52, published by: Bauverlag GmbH, Am Klingenweg 4a, D-65396 Walluf.
All three generic types referred to are based on different philosophies concerning the physical effects occurring during compaction. Even seemingly slight differences in features of the physical effects used may be of significance here, such as for example the forming of one and the same static moment on unbalanced bodies of unbalance vibrators with greater or smaller center-to-center spacings, associated with smaller or greater masses. All three generic types share the common feature that it is endeavored when operating the compacting devices to operate the oscillating systems in such a way that highest possible compacting accelerations are achieved in the molding material with highest possible oscillating frequencies (as far as possible to about 70 Hz), it also being intended that the accelerations and frequencies can be set according to values which can be given. In any event, the oscillating acceleration of the vibrating table always involved, on which not only the result of compaction but also the loads on the components involved depend, is a linear function of the oscillating amplitude and a square function of the oscillating frequency.
The publication EP 0 515 305 B1 describes a directional vibrator which can be adjusted with respect to the oscillating stroke amplitude (amplitude decisive here for the compacting acceleration) and the oscillating frequency, with 4 unbalanced shafts of a compacting device of the first generic type. The 4 unbalanced shafts are driven by a driving and adjusting motor of their own in each case, by way of universal shafts. The adjustment of the phase angle defining the oscillating stroke amplitude takes place exclusively by means of motor torques to be correspondingly set, which generate a reactive power in the case of a phase angle deviating from the value 0° or 180° (as also described for example in DE 40 00 011 C2). The following features are to be mentioned as disadvantages of such an unbalance vibrator and compacting method:
The uppermost oscillating frequency is generally restricted in practice to 50 Hz because of the constant loading limit to be taken into consideration, the limit loading being reached in particular when there are rolling bearings of the unbalanced shafts and the articulated shafts are co-oscillating. In this respect, see also the article in the specialist journal cited above on page 45, middle section, and on page 47, middle section.
High power losses occur due to the reactive power to be constantly converted and due to the high bearing friction energy levels generated when there are high centrifugal forces. Since the high power losses also have to be converted in the drive motors of the unbalanced shafts, the motors and their activating devices are dimensioned unnecessarily large with respect to the compacting power alone.
As a result of the masses of inertia to be overcome of the motors and unbalanced bodies and as a result of the fact that changing of the phase angle is also always accompanied at the same time by changing of the reactive power torque, likewise to be corrected along with it, the values of the phase angles given as a controlled variable (static moment) can only be regulated with rough tolerances by the electronic closed-loop control (or else by alternative mechanical controls), which leads to corresponding unevennesses of the oscillating stroke profile of the vibrating table during the compacting operation, proceeding over many oscillating periods, and consequently, to poor reproducibility of the compacting quality. Added to this here is the disadvantage that the rough tolerances of the “phase angle” controlled variable affect the relative angular position of a total of 4 unbalanced bodies, which usually lie with their axes of rotation in one plane and the arrangement of which extends over a large part of the longitudinal extent of the vibrating table. The dissimilarities of the relative angular positions leads to dissimilar accelerations with respect to the overall table surface. This leads in turn to dissimilar compacting results at different locations of the table surface.
The oscillating stroke amplitude of the vibrating table, decisive for the compacting effect, can be regulated only indirectly and sluggishly by means of the adjustable phase angle.
Apart from the masses of inertia, the regulating of the phase angle is made more difficult in principle by the fact that, when the vibrating table strikes against the pallet, the rotational velocity of the unbalanced shafts always experiences an abrupt change, the changes in velocity, and consequently angle of rotation, taking different values because of the relative position of the unbalanced bodies during the impact, dependent on the phase angle.
The regulating of the phase angle takes place by the rotational velocity of the unbalanced shafts being regulated in relation to one another. This means that simultaneous regulating of the phase angle and oscillating frequency cannot be achieved simultaneously in practice and can only be achieved with difficulty.
It is desired to be able to use a method in which, during the operation of main compaction, a given range of the compacting frequency up to highest frequencies is passed through with given values for the oscillating stroke amplitude of the vibrating table. In the case of this method, the micro-oscillating systems contained in the molding material and defined by the different grain sizes can be excited with different natural frequencies to produce resonance effects, whereby the compaction is improved. It must be possible in this case for the passing through of the frequency range to be carried out in about 3 seconds. In the case of the prior art, the implementation of this method is hindered by the limitation of the oscillation frequencies of the vibrating table and by the poor simultaneous controllability of the oscillating frequency and oscillating stroke amplitude.
The present invention is not suggested by the publications mentioned, DE 44 34 679 A1 or EP 0 870 585 A1, if only because they describe compacting devices which operate in a quite different way (shaking compaction and harmonic compaction, respectively) with different compacting mechanisms. The spring system of the vibrating table described in DE 44 34 679 cannot serve as a model insofar as a force transfer by the springs in both directions of oscillation is envisaged, since in the case of the spring system described spring elements 116 which operate simulataneously as compression springs and tension springs are provided. This means stress loading of the springs that is twice as high in comparison with a type of construction in which springs are only loaded by compression. What is more, the force connection of a spring loaded by compression and tension at its ends to a frame (or the foundation) of the compacting device on the one hand and to the vibrating table on the other hand is very problematical and cannot be sustained in the long term with a highly dynamic mode of operation envisaged here. The hydraulic exciter actuators shown in DE 44 34 679 must at the same time also undertake the function of a linear guide of the vibrating table. Since, with impact operation under the pallet, the vibrating table tends toward constantly changing inclined positions, this means high mechanical loading of the exciter actuators by the function allocated to them of linear guidance, which is further increased by the tendency toward jamming occurring in this case when there are two linear guides present.
The compacting device described by the publication EP 0 870 585 also cannot act as a model with respect to the following functions: the hydraulically designed system spring is able to execute a spring action only in the case of a downwardly directed oscillating movement and the use of the same fluid medium for the hydraulic exciter and for the hydraulic spring demonstrably leads to considerable energy losses also when executing the spring function. As disclosed by column 2, lines 25 to 30, the spring constant is evidently to be variable only for the purpose of adapting the compacting method to the masses of different sizes occurring in the case of products to be differently compacted, in order to re-establish the natural frequency of the mass-spring system, given as a fixed value. Changing of the natural frequency during the compacting operation is not envisaged.